Page 385 - Instrumentation Reference Book 3E

368 Chemical analysis: electrochemical techniques

- 0.2 -
c
D
CP
.o 'y
0 .- -
8 2 0.1
;E
18

I I

PH
Figure 17.24 Relationship of pH and Na ion error.
Courtesy Kent Industrial Measurements Ltd. Analytical
Instruments.
alkalinity. These are less prone to interference.
For a given glass electrode at a stated measuring
temperature the magnitude of the error can be
found from tables provided by electrode manu-
facturers. An example is shown in Figure 17.24. antimony
rod
Temperature errors The calibration slope Figure 17.25 Antimony electrode.
and standard potential of ion-selective electrodes
(including glass pH electrodes) are affected by Sb203, and the equilibria governing the electrode
temperature. If the pH is read directly off the potential are:
pH scale, some form of temperature correction
will be available. but often only for the calibra- Sb + Sb3+ + 3e-
tion slope and not for the standard potential. If [Sb3+]
measurements are made at a temperature differ- Sb203 + 6H+ 4 2Sb3+ + 3Hz0, K = ~ F+I3
ent from that at which the electrode was cali- However, there are many possible side reac-
brated there will be an error. This will be small tions depending on the pH and the oxidizing
if the meter has an iso-potential setting. For the conditions; salt effects are large. There is there-
most accurate work the sample and buffer solu- fore difficulty in calibrating with buffer solutions;
tions should be at the same temperature, even if stirring temperature and the amount of oxygen
iso-potential correction is possible.
present all have rather large effects. A reproduci-
bility of about 0.1 pH unit is the best that is
Stirring factor In well-buffered solutions it may normally attained, the response is close to Nerns-
not be necessary to stir when making pH meas- tian over the pH range 2 to 7, and the response
urements. However, it is essential in poorly buf- time can be as short as 3 minutes but is often
fered solutions. about 30 minutes.
The outstanding advantage of the antimony
electrode is its ruggedness, and for this reason it
The Iiydrogen electrode The hydrogen elec- has been used for determining the pH of soils.
trode, consisting in practice of a platinum plate Also, of course, it is indispensable for solutions
or wire coated with platinum block (a finely containing hydrofluoric acid which attack glass.
divided form of the metal) can measure hydrogen
ion activity when hydrogen is passed over the If the electrode becomes coated during use, its
performance can be restored by grinding and
electrode. However this electrode is neither easy polishing the active surface and then reforming
nor convenient to use in practice and is now never
used in industrial laboratories or on plant. the oxide film by immersion in oxygenated water
before using in deoxygenated solutions.
However, there is much more uncertainty to
The aiitiinony electrode The antimony electrode every aspect of behavior of the antimony elec-
is simply a piece of pure antimony rod (- 12 mm trode than with the glass electrode and even the
diameter, 140mm long), housed in a protective fragile glass electrodes of years ago with their
plastic body resistant to acid attack; see Figure limited alkaline range displaced the antimony
17.25. The protruding antimony rod when electrode when accurate pH measurements were
immersed in a solution containing dissolved oxy- required. Modern glass electrodes are excellent in
gen becomes coated with antimony trioxide respect of robustness and range, and antimony